The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
Many modern motor vehicle automatic continuously variable transmissions (CVT) utilize controlled hydraulic fluid (transmission oil) to actuate CVT belt and pulleys (or chain and pulleys) to achieve a desired ratio in downsized turbo boosted engines to optimize fuel economy. The control of such hydraulic fluid is achieved by a valve body which comprises a plurality of spool valves which direct hydraulic fluid flow through a complex of passageways to CVT pulley pistons as well as other clutch and brake actuators. The valve body is supplied with pressurized hydraulic fluid from, typically, a gear or vane pump, which is driven by the engine output shaft or the transmission input shaft.
Because this is such a common transmission configuration and because of the manufacturing volume of such automatic transmissions, extensive research and development has been undertaken to reduce the cost and optimize the performance of such pumps. For example, simplifying such pumps to reduce their weight and cost, reducing their size to improve packaging, improving low speed performance, improving low temperature performance and reducing high speed energy losses have all been areas of development and improvement.
A fixed displacement pump provides flow proportional to engine speed. The pump is often sized to meet hydraulic pressure and volume demands of the transmission at low speed idle engine conditions. Friction forces inside the pump increase as the size of the surface area of the pump rotor increases. Accordingly, larger diameter higher displacement pumps that meet hydraulic demands of the transmission near engine idle speed often contribute to undesirable transmission spin losses and decrease efficiency of the transmission. A large pump will provide much greater oil flow than what is consumed by the transmission at higher engine speeds, with higher pump power consumption leading to loss in overall transmission efficiency.
The present invention is directed to improvements that reduce pump spin losses and improve transmission efficiency while meeting hydraulic demands of the transmission.